As a rule, the intensity of the current flowing through a load in a motor vehicle is determined in that the voltage, which drops across a measurement resistor disposed in the load current circuit, is amplified and the current is computed with the aid of the known amplification and the known resistance value. The amplified voltage measurement value is, however, corrected in advance of this computation. The reason for this is explained based on a known evaluation circuit as it is shown in FIG. 4 of the drawings to which reference will be made below.
The evaluation circuit 10 shown in FIG. 4 includes two load outputs LA1 and LA2 across which a load L is connected. The evaluation circuit 10 also includes two signal inputs SE1 and SE2 and two signal outputs SA1 and SA2. In addition, a terminal for the battery voltage U.sub.-- BATT and a terminal for the reference potential BP are shown. Additional terminals such as those for the voltage supply of active components in the evaluation circuit 10 are not shown because they are not of significance for the following explanation.
The load current circuit extends from the terminal for the battery voltage U.sub.-- BATT via a switch, a measuring resistor R.sub.-- M (having a resistance value R.sub.-- M), the load terminal LA1, the load L, the load terminal LA2, a driver transistor T to the reference potential. The driver transistor T is driven via a drive circuit 11 which receives its drive signal from the signal input SE2. The switch S is opened and closed with the aid of a drive signal which is applied to the signal input SE1.
The voltage at the measuring resistor R.sub.-- M is supplied to an operational amplifier OP having an output which is connected to the signal output terminal SA2. The voltage at the connecting point between the switch S and the measuring resistor R.sub.-- M is connected to the other signal output SA1. The potential of the operational amplifier OP is adjustable via a bias voltage source U.sub.-- BIAS (having the voltage U.sub.-- BIAS). The operational amplifier has the amplification G. The following equation applies to the voltage between the reference potential and the output SA2 of the operational amplifier: ##EQU1##
In this equation, the value .DELTA.R.sub.-- CM.cndot.U.sub.-- BATT represents a voltage error which originates because of inadequate common-mode balancing of the operational amplifier. The inadequate common-mode balancing is dependent upon temperature and deterioration.
To calibrate the circuit of FIG. 4, it is conventional to apply a signal having a pregiven shape to the operational amplifier OP and to trim at least one resistor in the external circuit (of the operational amplifier) so that the output signal indicates a common-mode balancing as complete as possible.
In this way, the term in the above equation (1), which is dependent upon battery voltage, has the value zero. Thereafter, the bias voltage U.sub.-- BIAS is so adjusted that a positive but yet low output voltage of the operational amplifier is adjusted notwithstanding the offset voltage and even for a possibly occurring voltage, which is dependent upon temperature and deterioration, based on inadequate common-mode balancing at zero current and at very low currents. In this way, the entire swing of a downstream analog-to-digital converter can be utilized.
The common-mode balancing undertaken during the calibration operation can shift in dependence upon temperature and/or deterioration. When this occurs, it is a disadvantage of this method that the battery voltage operates with an adulterating influence on the measured value. Furthermore, at least two resistors must be trimmed, namely, one for the common-mode balancing and one for setting the bias voltage.